1. Field of the Invention
The present invention relates to a wiring board comprising a patterned wiring formed of conductive resin mainly composed of silver on a substrate. The invention also relates to a method of manufacturing the wiring board.
2. Background Art
A wiring board hitherto known has a patterned wiring composed of silver paste formed on a substrate by using a printing method such as screen printing. However, the patterned wiring formed of silver paste has a drawback that silver in the patterned wiring migrates and a resistance of a wiring conductor changes when it is wet with absorption of moisture or adhesion of water, or when there is an electric field. This is because the silver is prone to migrate as compared with other electrically conductive metals although it is an excellent conductive metal having low electrical resistance. For this reason, there have been heretofore proposed numerous ideas to prevent the silver migration. Since electrodes of a terminal portion need to be exposed for connection of the wiring board with an external apparatus, silver migration tends to occur especially in the terminal portion. Prevention of the migration is therefore desired strongly for the terminal portion.
In the case of using ordinary paste made of conductive resin containing silver particles, a finished patterned wiring has a considerably asperate surface after the paste is printed and hardened. It therefore has a problem that contact resistance can not be made small in a structure in which the wiring board is connected to an external apparatus through mechanical contact of the terminal portion.
A structure shown in FIG. 8 is disclosed in Japanese Patent Laid-open Publication, No. 2000-307207, as an example of a wiring board structure contrived to prevent silver migration. FIG. 8 is a perspective view showing a terminal configuration of a conventional wiring board for connection with an external apparatus. Each of terminal portions 200 comprises silver paste layer 220 made of silver paste printed on a surface of plastic film 210, and carbon paste layer 230 made of carbon paste printed thereon. A portion of plastic film 210 is cut off between terminal portions 200 to facilitate the connection. In addition, protective film 240 is formed on top of carbon paste layer 230 in an area other than terminal portions 200. Accordingly, silver migration due to moisture absorption or adhesion of water is prevented by this double-layered structure in which silver paste layer 220 is covered with lamination of carbon paste layer 230 which is not likely to migrate easily.
There is also a method of reducing contact resistance in the connecting portion such as the method disclosed in Japanese Patent Laid-open Publication, No. S58-134496, for example. That is, a surface of a patterned wiring formed of silver paste printed on a substrate such as a plastic film is smoothed by means of rolling or buffing to increase a contact area and decrease the contact resistance.
However, in the case of this double-layered structure in which the carbon paste layer is formed on the silver paste layer, side surfaces of the silver paste layer are not covered with the carbon paste layer. Because the silver paste layer is exposed at the side surfaces, this structure is unable to positively prevent the silver migration from the side surfaces. This structure also has an additional problem that formation of the patterned wiring of fine pitches is practically inhibitive, since the narrower a distance between conductors of the patterned wiring the more intense it becomes in the strength of the electrical field, which promotes the silver to migrate. Besides, the above publication does not make any reference to use of carbon paste for smoothing out asperities in a surface of the silver paste layer.
The conventional method of smoothing the surface of the silver paste layer by using a roller and the like is effective for reduction of contact resistance in the connection between the terminal portion of the patterned wiring and an external apparatus. However, it also has a problem that it is unable to prevent a failure caused by the silver migration because it is not provided with any measures to protect against the silver migration.